Effect of greywater treated by horizontal subsurface flow wetlands planted with Chrysopogon zizanioides and Andropogon gayanus on the germination of tomato (Lycopersicon esculentum Mill.) seeds under Sahelian climate

This study evaluates greywater treatment capacity by Horizontal Subsurface Flow wetlands (HSSF) planted with Chrysopogon zizanioides and Andropogon gayanus, and investigates the suitability of the effluents on the germination of tomato seeds. Three pilot-scale HSSF systems were designed: one planted with Chrysopogon zizanioides (HSSF-C), one planted with Andropogon gayanus (HSSF-A), and one unplanted (HSSF-UP). Principal component analysis showed that the planted systems provided higher removal efficiencies than the unplanted system. HSSF-A showed the best performance for removal of organic matter, including chemical oxygen demand (COD) and biochemical oxygen demand (BOD5), as well as mineral matter, including nitrate (NO3−) and ammonia (NH4+). In contrast, HSSF-C showed the best performance for removal of phosphorus compounds and fecal indicators, including thermotolerant coliforms and E. coli. HSSF-UP showed the lowest performance for removal of all parameters. The reduction of thermotolerant coliforms and E. coli was significantly greater in the two planted systems, exceeding 2 log10 removal. However, no significant difference was observed between the planted and unplanted HSSF for fecal indicators. The germination rates obtained with greywater treated by HSSF were higher than 80%, approaching that obtained using distilled water. In contrast, lower germination rates were observed with seeds irrigated with raw greywater (58%). This indicates the beneficial effect of treated greywater by HSSF wetlands on seeds germination. The residual content of fecal indicators and physicochemical characteristics of the treated greywater were compliant with World Health Organization (WHO) reuse guidelines for restricted irrigation. The results demonstrate the possibility of using constructed wetlands for greywater treatment and its reuse in gardening in Sahelian countries. The study suggests that HSSF wetlands planted with Chrysopogon zizanioides or Andropogon gayanus can effectively treat greywater, reducing the risk of waterborne pathogens and improving the suitability of the effluents for irrigation.